The present invention relates to a process for the production of acrolein by catalytic gas-phase oxidation of propene with air in the presence of steam and an inert gas or, preferably, waste gas from the reaction, from which the condensible constituents have been removed, at elevated temperature and in a ratio of propene to air to inert gas or waste gas to water of 1:6-9:3-12:0-5.
The highly exothermic reaction of propene on heterogeneous catalysts with an oxygen-containing gas leads to a number of unwanted secondary products in addition to the desired product acrolein. It is known that local overheating of the catalyst and the resulting increased formation of secondary products can be avoided by effective dissipation of the heat of reaction, for example in tube bundle reactors.
It is also known that the pressure loss of a catalyst bed can be influenced by the size and external shape of the catalyst elements. The internal structure of the catalyst elements (porosity, length of the diffusion paths) critically determines mass transfer and heat transfer in the catalyst and, accordingly, has a major bearing on selectivity along with the composition of the catalytically active mass. High compressive strength and abrasion resistance are requirements which a catalyst has to satisfy for use on an industrial scale. High abrasion would mean that, during filling of the tubes of a tube bundle reactor, the scattering of the pressure losses of the individual tubes would be high, resulting in different throughflow rates and impaired selectivity.
DE-PS 31 25 061 describes a process for the production of acrolein using shell catalysts. With shell catalysts, local overheating is avoided by the temperature-equalizing effect of the inert support; in the relatively thin shell, the diffusion paths for the gaseous reactants are short.
DE-OS 33 38 380 describes catalysts for the oxidation of propene to acrolein which are made in the form of rings or hollow cylinders from a mass containing Mo, Fe, Bi and W. These catalysts may be assumed to have been derived from shell catalysts by replacement of the inert core of the shell catalysts by an "inert void", the shell being open on two opposite sides to allow the reactants access to the void. Compared with shell catalysts, these ring or hollow cylinder catalysts have a larger ratio of outer surface to volume. The active mass is thus more accessible to the reactants. The low pressure loss and high heat dissipation of the shell catalysts are again present. To ensure that the "hollow catalysts" have sufficient mechanical strength, the active mass is highly compressed, with the result that the inner structure is adversely affected.
EP-OS 0 184 790 describes ring-shaped catalysts, albeit with rounded end faces to improve fillability, but does not mention either the catalyst mass or a special production process; in particular, there is no reference to measures for producing a special, favorable inner structure.
In order to enable the active mass to be optimally utilized, the inner structure of the catalyst has to be such that a potentially high reaction velocity is not limited by obstacles to mass transfer within the catalyst. An attempt along these lines is described in EP-OS 0 279 374 which relates to a process for the production of a catalyst containing Mo, Fe, Bi which is characterized by its specific surface, pore volume and pore distribution. On account of the production process, however, it is only possible to obtain catalyst particles substantially spherical in shape, i.e. with a very small ratio of surface to volume, or the particles would have to be very small. So far as industrial application is concerned, however, there are limits to this on account of the high pressure loss involved.
The catalysts produced and used by known methods have certain disadvantages in regard to the aspects mentioned. Elements of different shape are used with a view to shortening the diffusion paths, avoiding local overheating or achieving better utilization of the catalyst volumes by a suitable inner structure of the catalyst. Hitherto, individual measures along these lines have resulted in comparatively unsatisfactory productivity per catalyst volume used where catalysts of the type in question are used in the industrial production of acrolein. This is a considerable economic disadvantage because, by way of compensation, large and expensive reactors with a high filling volume for the catalysts have to be used to carry out the reaction.
One of the objects of the present invention was to provide an improved process for the production of acrolein by catalytic gas-phase oxidation of propene with air which would operate in known manner in the presence of steam and an inert gas or, preferably, waste gas from the reaction from which the condensible constituents have been removed, would use higher temperatures and in which the ratio of propene to air to inert gas or waste gas and water was 1:6-9:3-12:0-5.